5.56 versus .223 - What's The Difference?

.223 Remington on the left, and M855 5.56mm on the right ammo.

You made the decision to be a citizen, got lucky and scored yourself that AR-15 you’ve been eyeing since the Chinese Flu panic broke out. Urban unrest hustled things along of course. Now you gotta feed your black rifle. Chances are your local gun shop led you to some offerings from Federal or Horandy. But those 100 rounds or so aren’t enough, especially these days. So now you’re hitting the interwebs, looking high and low for ammo, any ammo. Inventory is paltry, but you’ll see varying quantities of 5.56mm and .223 - they look the same, and your rifle can probably eat both - but there’s differences. What are they? And more importantly, is it safe to run both? Read on…

My Misconceptions Were Borne Of Convenience

Many years ago now, I got my first AR-pattern rifle. It was a Colt LE6920, regarded as the “benchmark” of AR-15s that a citizen can easily acquire. The rollmark (the engraving on the magazine well) said “Cal. 5.56mm”, so I purchased and ran that exclusively. Plus, the received wisdom told me that .223 was categorically inferior to 5.56mm and was to be avoided by anyone running a rifle built to military specifications or better. On a lark, I ran some cheap-as-nuts Tula (Russian) .223 and it was jam-o-matic - so yeah, I avoided .223 in my ARs.

Fast forward a few years, and I started becoming a little more thorough in my purchasing and research, and found that it wasn’t necessarily a bad thing to run .223 ammunition - but first, I had to educate myself as to what I was doing - like with anything related to firearms.

To start, we’ll have to understand the basic differences between the two calibers.

How Are 5.56mm And .223 Ammunition Different?

Like a lot of things in the gun world, the development of both 5.56x45mm (the 45mm refers to the length of the casing) and .223 Remington started a long time ago. .223 started it’s development in 1957 for what would become known as the AR-15 rifle. Designed by Remington, back when they actually designed things, the cartridge is actually descended from the .222 Remington, a cartridge designed from the get-go for the sporting market. .222 is actually still prized for it’s accuracy amongst collectors. Anyway, after some engineering work, the .223 was born, and Eugene Stoner had his ammunition for his rifle, the AR-15.

After many years of politics, acceptance trials, revisions, and business deals, the military finally accepts the AR-15 and .223 Remington cartridges for service, calling them the M16 and M193 respectively. This all came to a head in 1964. If you really want to take a deep dive, check out Sniper Country’s extremely detailed history of the AR-15/M16, along with an amazing amount of technical data.

Along the way, SAAMI - the Sporting Arms and Ammunition Manufacturers’ Institute, which is the US body that standardizes ammunition, accepted .223 Remington as a standard in 1962. Which basically means that for a cartridge to call itself .223, it has to conform to the SAAMI standards, which have not changed markedly since then.

But what about 5.56mm?

Unlike .223 Remington, 5.56mm is not a standard outside of the military. In the late 1960s, the British were looking for a smaller, higher-velocity round for their own needs, and since they were a part of NATO, they looked to have a standard round to allow interoperability with allied forces, especially the US. On the table was the M193 round, of course. Since this was Europe, where metric is king, the round was known as 5.56x45mm. The NATO committees had their reservations though, since the M193, with it’s rather light 55-grain projectile, and M16 were having some issues in the battlefield, since by then, the US was committed to the Vietnam War.

At the typical glacial pace of committees, it took until 1977 for NATO to even begin field testing M16s, along with a new cartridge based on the M193, the SS109. Refined by the Belgians, the SS109, known as the M855 in the US, became “the” standard ammunition for NATO countries. The SS109/M855 cartridge sports a 62-grain projectile with a mild steel core, capable of penetrating an M1 battle helmet at 800 meters. Us Regular Guys and Girls know the M855 as “green-tip” ammo due to it’s distinctive green-painted point.

So, what are the differences?

To sum it all up:

One difference is the higher pressure level of the 5.56x45mm NATO cartridge which runs at approximately 58,000 psi. A .223 Remington is loaded to approximately 55,000 psi. So you’re getting more pressure out of 5.56mm, which usually translates to more velocity.
The second and most important difference between the two is the fact that a 5.56 NATO chamber has a .125” longer throat. This allows approximately one more grain of powder to be loaded into a 5.56 NATO cartridge, giving it that extra “oomph”.
Dimensionally, both rounds are identical, and will load in each other’s chamber.

In these differences lies the danger though.

5.56mm Rifles Can Run .223 Remington, But Not Vice-Versa

Just to put your mind at ease, your AR-pattern rifle is most likely chambered in 5.56mm. For logistics and marketing reasons, most standard AR-15s these days ship chambered in 5.56mm. Logistically because most professional and government users demand the spec, and also from a marketing standpoint, 5.56mm is “mil-spec”, and everyone wants a rifle that operators operating operationally will use. No wimpy .223 need apply, nosireebob.

The quickest way to check what your rifle is chambered in is to look to the rollmark. The rollmark will usually specify what caliber is safe for your rifle to use. In the case of an AR-15, it will say “5.56mm”, .”.223” or “5.56/.223”.

Note, if your rifle rollmark says “Cal Multi” or “Multi-Cal”, then most likely your rifle was built by hand and not at the factory, so your next visual cue is on the barrel itself, which is the definitive visual cue for caliber. Most likely it will say “5.56 NATO 1/7” or “.223 Rem”. You might see “.223 Wylde” - we’ll go over that later.

The identification markings on a 5.56mm barrel by Radical Firearms.

Again, you most likely have a 5.56mm-chambered rifle. You can safely run .223 Remington ammo, as well as 5.56x45mm NATO ammo. The military spec was designed with enough tolerance to “eat” both, and do it dirty as well. The only downside of running .223 is you’ll lose some accuracy since the round won’t achieve it’s optimal pressure. In a citizen-standard 16-inch barrel this is inconsequential. Where you’ll run into problems is if you have an short-barreled build (pistol or rifle), where the lower pressures mean less gas is coming back, thus the weapon may not cycle properly. If you’re running “short”, stick to 5.56mm for reliability reasons.

Where the danger crops up is if you’re in possession of a rifle explicitly chambered in .223 Remington, and you decide to run 5.56mm. The round, expecting that ever-so-slightly longer throat, will fit that much tighter into a .223 chamber. What this means is the 5.56mm round will make the chamber pressure spike up to 65,000 PSI, which is 10,000 PSI higher than what .223 is rated for. You might get away with it, once or twice, but the results could prove inconvenient at best, or damaging at worst. The increased pressure could cause the ammunition’s primer to back out, getting jammed up in the trigger mechanism, or result in damage to the barrel or chamber, which could eventually lead to a catastrophic failure over a course of fire.

Fortunately, .223 rifles sold as-new nowadays are rather rare. Still, it’s good practice to check the specifics before you buy.

While shopping for a shiny (or matte) black rifle, you might see the specification “.223 Wylde“ pop up. This isn’t some sort of new ammo spec, it’s actually a specification for a chamber. Developed by Bill Wylde in the 1980s, the goal was to provide a chamber which accepted both .223 and 5.56mm and did not sacrifice accuracy for either. Bill Wylde produced competition and match rifles, so accuracy across a broad spectrum of ammo was very important. .223 Wylde didn’t catch on outside of competition circles until recently, but nowadays, it’s not too difficult to find an AR with this chamber. If your AR says .223 Wylde, you are good to go with both .223 Remington and 5.56x45mm NATO, and you’ll enjoy optimal performance out of both.

My Recommendations

Chances are, your AR-15 is chambered in 5.56x45mm NATO. As I said above, for logistical and marketing reasons, it’s the most common these days. It’s also the “safest bet” that is commonly available. You actually have to work to find a .223 chambered AR, and usually it will be a vintage piece or something severely discounted. Stick with 5.56, or .223 Wylde if you come across it.

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